The hydrodynamic properties of the shell of Chlamys opercularis have been investigated and compared with other Pectinidae. The coefficient of lift increases with increasing current speed and angle of attack. Little benefit is gained by increasing the angle of attack beyond 20 °. Allometric changes in shell shape are not sufficient to counteract the increasing weight of the animal with growth. Flow patterns over the shell indicate that a smooth and attached flow over the body of the shell becomes broken up into a turbulent, eddying wake at angles of attack beyond 20 ° and at current speeds greater than 25 cm s−1. The effect of the auricles is to enhance lift at low speeds and to help maintain stability. It is postulated that large turbulence may be further reduced if the dorsal jets act as jet flaps.